The present invention relates to a radiation image capturing apparatus, particularly to a radiation image capturing apparatus wherein the apparatus performs radiation image capturing operations by detecting irradiation by itself.
There has been development of various types of radiation image capturing apparatuses including a so-called direct type radiation image capturing apparatus that generates an electric charge through a detection element in response to the dosage of applied radiation such as X-rays and converts the electric charge into an electric signal, and a so-called indirect radiation image capturing apparatus that uses a scintillator etc. to convert the applied radiation into electromagnetic waves having other wavelengths such as visible light, then generates an electric charge through a photoelectric conversion element such as a photodiode in response to the energy of the electromagnetic wave having been converted and applied, and converts the electric charge into an electric signal (i.e., image data). In the following description of embodiments of the present invention, the detection element in the direct type radiation image capturing apparatus and the photoelectric conversion element in the indirect radiation image capturing apparatus will be collectively referred to as a radiation detection element.
This type of radiation image capturing apparatus is known under the name of FPD (Flat Panel Detector). In the conventional art, this radiation image capturing apparatus has been designed as a so-called exclusive device type formed integrally with the support base (refer to the Unexamined Japanese Patent Application Publication No. Hei 9 (1997)-73144, for example). In recent years, there has been development of portable radiation image capturing apparatuses wherein a radiation detection element and others are incorporated in a casing for easy transportation. These portable radiation image capturing apparatuses have been put into practical use (refer to Unexamined Japanese Patent Application Publication No. 2006-058124, and Unexamined Japanese Patent Application Publication No. Hei 6 (1994)-342099).
In the aforementioned radiation image capturing apparatus, a plurality of radiation detection elements 7 are normally arranged in a two-dimensional array (matrix) on a detecting section P, and each radiation detection element 7 is connected with the switch unit formed of a thin film transistor (hereinafter referred to as “TFT”) 8, as shown in FIG. 7 to be described later.
Normally in the radiation image capturing operation, radiation is applied to a radiation image capturing apparatus from a radiation source of a radiation generator, with a prescribed image capturing position (front of the chest or side of the lumbar spine) of the body of a subject placed in-between.
In this case, off-voltage is applied to the lines L1 through Lx of the scanning line 5 from the gate driver 15b of the scanning drive unit 15 of the radiation image capturing apparatus. When all the TFTs 8 have been set to the off-state, radiation is applied, whereby an electric charge generated within each radiation detection element 7 by application of radiation is stored appropriately inside each radiation detection element 7.
After radiation image capturing operation, on-voltage is applied sequentially to each of the lines L1 through Lx of the scanning line 5 from the gate driver 15b so that TFTs 8 are sequentially turned on. The electric charge accumulated in each radiation detection element 7 by application of radiation is sequentially discharged to each of the signal lines 6. This electric charge is then read out as image data D by each reading circuit 17.
Incidentally, to ensure radiation image capturing, it is required that, when radiation is applied to the radiation image capturing apparatus, off-voltage should be properly applied to each of the lines L1 through Lx of the scanning line 5 from the gate driver 15b, and TFTs 8 as switch unit should be turned off, as described above.
In many of the conventional exclusive equipment type radiation image capturing apparatuses, for example, an interface is provided for connection with the radiation generator so that signals are exchanged. Then the radiation image capturing apparatus applies off-voltage to each of the lines L1 through Lx of the scanning line 5. When the charge accumulation state has been confirmed, the radiation image capturing apparatus allows radiation to be applied from the radiation source.
However, for example, when the radiation image capturing apparatus and radiation generator have been produced by different manufacturers, it is not always easy to provide interface between these devices. In some cases, an interface cannot be provided.
If an interface cannot be configured between the radiation image capturing apparatus and radiation generator, the radiation image capturing apparatus has no means of identifying the time when radiation was applied from the radiation source. This requires the radiation image capturing apparatus to detect by itself whether or not radiation has been applied from the radiation source.
To solve this problem, in recent years, it has been known that development of various radiation image capturing apparatuses capable of self-detection of the application of radiation, independently of the aforementioned interface configured between the radiation image capturing apparatus and radiation generator.
For example, according to the inventions proposed in the Specification of the U.S. Pat. No. 7,211,803 and the Unexamined Japanese Patent Application Publication No. 2009-219538, when exposure of the radiation image capturing apparatus to radiation has started, and electric charge has been generated inside each radiation detection element 7, electric charge flows from each radiation detection element 7 to the bias line 9 (refer to FIG. 7 to be described later) connected to each radiation detection element 7, with the result that there is an increase in the volume of current running through the bias line 9. It is proposed that to utilize this phenomenon effectively, the bias line 9 is provided with a current detection unit to detect the value of the current flowing through the bias line 9 and that thus, the start of irradiation is detected based on this current value.
According to the research made by the present inventors, however, it has been found out that since the aforementioned technique uses a bias line 9 connected to the electrode of each radiation detection element 7, noise generated by the current detection unit is transmitted to each radiation detection element 7 through the bias line 9, and is superimposed on the image data D read out of the radiation detection element 7 in some cases and that solution to the problem is not easy.
In the meantime, after extended research on an alternative method that enables the start of irradiation to be detected by the radiation image capturing apparatus, the present inventors have found out several techniques that enable the radiation image capturing apparatus to detect the start of irradiation appropriately by itself.
As will be described later, a new irradiation start detection method found out by the present inventors is designed in such a way that, prior to radiation image capturing operation, on-voltage is sequentially applied to each of the lines L1 through Lx of the scanning line 5 from the gate driver 15b of the scanning drive unit 15 so that image data “d” is read out. It should be noted that, in the following description, the image data to be read for detection of the start of irradiation prior to this radiation image capturing will be called image data “d” for irradiation start detection, for distinction from the image data D which is a main image to be read immediately after image capturing.
When radiation is applied to the radiation image capturing apparatus, there is an increase in the value of the image data “d” for irradiation start detection to be read. This phenomenon is used in such a way that the start of irradiation to the radiation image capturing apparatus is detected based on the image data “d” for irradiation start detection having been read.
Further, in the another irradiation start detection method found out by the present inventors, off-voltage is applied to all the scanning lines 5 from the gate driver 15b of the scanning drive unit 15 prior to radiation image capturing so that each of the TFTs 8 is turned off. Under this condition, the reading circuit 17 is made to perform the step of reading. Then the step of reading leak data “d leak” is performed in such a way that the electric charge “q” (refer to FIG. 13 to be described later) having leaked from the radiation detection element 7 through the TFTs 8 is converted into the leak data “d leak”.
In this case as well, when radiation has been applied to the radiation image capturing apparatus, there is an increase in the value of the leak data “d leak” to be read. This phenomenon is utilized so that the start of irradiation of the radiation image capturing apparatus is detected based on the value of the leak data “d leak” having been read.
In this case, the above-mentioned leak data “d leak” or the image data “d” for detecting the start of irradiation are provided with threshold value “d leak_th” or threshold value “dth” in advance. When the read-out leak data “d leak” or image data “d” has exceeded the threshold value “d leak_th” or “dth”, start of irradiation on the radiation image capturing apparatus is detected.
However, the further study by the present inventors has revealed that periodic fluctuations are sometimes found in the values of the leak data “d leak” or the image data “d” for detecting the start of irradiation read out by the radiation image capturing apparatus prior to radiation image capturing operation. It has also been found out that such periodic fluctuations in the read-out value of the leak data “d leak” or image data “d” may allow the threshold value “d leak_th” or “d th” to be exceeded even though radiation is not applied to the radiation image capturing apparatus when there is a large fluctuation in the leak data “d leak” or image data “d”. This may lead to incorrect detection of the start of irradiation.
As described above, when the radiation image capturing apparatus itself detects the start of irradiation based on the leak data “d leak” or image data “d” for detecting the start of irradiation, the radiation image capturing apparatus subsequent to the start of irradiation automatically transfers to the state of electric charge accumulation wherein the electric charge generated in the radiation detection element 7 by the irradiation is accumulated in the radiation detection element 7, as will be described later. After that, a step is taken to read out the image data D as the main image from each radiation detection element 7.
As described above, if there is an incorrect detection of the start of irradiation by the radiation image capturing apparatus, a series of processing such as transfer to the state of electric charge accumulation and reading of the image data D as the main image is performed automatically. Thus, for example, when a radiographing technician wishes to use this radiation image capturing apparatus to perform radiation image capturing operation, he cannot perform capturing operation using the radiation image capturing apparatus until completion of the above-mentioned series of processing.
Further, when the radiation image capturing apparatus has a battery built therein, power will be consumed by incorrect detection in the processing such as transfer to the state of electric charge accumulation and reading of the image data D as the main image. This may result in unwanted consumption of battery, and may reduce the number of radiation images captured in one step of battery charging. This may cause a reduction in the image capturing efficiency for each charging operation.